Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
An electromagnetic-induction heating type fixing device generates magnetic flux with an excitation coil so as to cause an eddy current in an induction heating layer provided within a heating member. The eddy current generates Joule heat to heat the induction heating layer, thus heating the heating member to a predetermined fixing temperature. This type of fixing device can reduce the thermal capacity of the induction heating layer to shorten warm-up time for starting the device, and to ensure a compact device and high thermal conversion efficiency. However, in cases where a printing sheet to be fixing processed is of small size, in the sheet-passing region through which printing sheets pass, the printing sheets absorb heat from the surface of the heating member such that the heating member loses heat. On the other hand, the sheet non-passing regions through which printing sheets do not pass is in a high-temperature state. Especially in the case where printing sheets pass through continuously, when the sheet-passing region of the heating member is maintained at the fixing temperature, the temperature of the sheet non-passing region of the heating member rises excessively, such that the temperatures of the heating member and the excitation coil exceed their heat-resistance limit temperatures. This can result in drawbacks such as thermal failure of these components.
Fixing devices have been proposed to solve the above-described drawbacks. For example, one fixing device includes a coil, a magnetic core, and a shielding member. The coil generates magnetic flux for induction heating of the heating member. The magnetic core is disposed on the opposite side of the coil from the heating member, and surrounds the coil. The shielding member is disposed between the coil and the magnetic core in a position facing the sheet non-passing region for smaller-size printing sheets so as to shield the path of the magnetic flux. During a fixing process on a printing sheet with the maximum sheet-passing width, the shielding member is located at a magnetic path freeing position in the wound part of the coil. During a fixing process on a printing sheet with a small width, the shielding member moves into a magnetic path shielding position, which is at the center of the coil windings. Disposing the shielding member in the magnetic path shielding position during the fixing process on printing sheets of small width weakens by means of the shielding member the magnetic flux acting on the sheet non-passing region. This reduces heat generation in the heating member in the sheet non-passing region.
Another exemplary fixing device includes a coil, which generates magnetic flux for induction heating of the inner periphery of a heating roller, and a flexible shielding member between the coil and the inner peripheral surface of the heating roller. The heating roller has an end on which a winding roller, which winds up and houses the shielding member, is disposed. During a fixing process on a printing sheet of maximum sheet-passing width, the shielding member is wound in by the winding roller and then housed. During a fixing process on printing sheets of small width, the shielding member is arranged to shield a part of the magnetic flux from the coil toward sheet-passing portion of the heating roller. This weakens by means of the shielding member the magnetic flux acting on the sheet non-passing region, thus reducing heat generation in the sheet non-passing regions of the heating roller.
Another exemplary fixing device includes a coil, a first magnetic core, and a second magnetic core. The coil generates magnetic flux for induction heating of a heating member. The first magnetic core surrounds the coil and forms a magnetic path. The second magnetic core is disposed in the hollow portion of a loop-shaped coil, and forms a magnetic path. The second magnetic core is rotatable and has a cylindrical cross section. A shielding member is mounted on the outer peripheral surface of the second magnetic core. At a position in a portion of the second magnetic core along its periphery, the shielding member is disposed facing a sheet non-passing region to the outer side of the sheet-passing region of printing sheets of small width. During a fixing process on a printing sheet with the maximum sheet-passing width, the shielding member moves into a position most separated from the heating member. During a fixing process on a printing sheet of small width, rotation of the second magnetic core moves the shielding member into a shielding position close to the surface of the heating member. Arranging the shielding member in the shielding position during the fixing process on paper sheets of small width weakens by means of the shielding member the magnetic flux acting on the non-paper passing region, thus reducing heat generation in the non-paper passing region of the heating member.
Further, another exemplary fixing device includes a loop-shaped coil, a first magnetic core, and a second magnetic core. The coil generates magnetic flux for induction heating of a heating member. The first magnetic core surrounds the coil and forms a magnetic path. The second magnetic core is disposed in the hollow portion of the coil, and forms a magnetic path. An endless belt is suspended between the second magnetic core, which is rectangular in cross section, and a rotationally driving roller. On a surface of the belt, a shielding member that shields the path of the magnetic flux is disposed. The shielding member covers over an area of the second magnetic core corresponding to a sheet non-passing region to the outer side of the sheet-passing region of printing sheets of small width. Arranging the shielding member in the magnetic path between the second magnetic core and the heating member during a fixing process on a printing sheet of small width weakens by means of the shielding member the magnetic flux acting on the sheet non-passing region, thus reducing heat generation in the non-paper passing region of the heating member.